Welcome Powermonkey.
Your safety can never be guaranteed in the Berg.
Even big groups are vulnerable to the petty, opportunistic theft that typically happens at night (which is the most common security issue). Larger groups definitely have an advantage when it comes to warding off a more confrontational attack. But if someone is going to go out their way to mug you, the advantage of two guys in a hiking group vs a guy and a girl is probably not that pronounced. Where this does leave you vulnerable is when it comes to sexually oriented attack. I only know of rape or attempted rape cases in the Giants Castle area however, and the last incident was in 2006.
You need to be aware that the Bell Traverse is used by the Basotho and locals from the Mnweni alike. Read through
this link, among others. You may have heard about the recent fatal accident at the base of Cathedral Peak (see Dragon's Wrath 2010 thread), and while there is no conclusive evidence that it was a security related accident, the incident has unsettled the Berg hiking community somewhat. Between Mlambonja Pass and Tseketseke Pass you might encounter the odd shepherd and I consider that stretch to be more relaxed than the BT.
Consider not sleeping in Bell Cave and Twins Cave - this would reduce the chances significantly. Unfortunately these caves are part of the classic experience of the BT. And the fact that you are posing this question indicates an awareness which will go far in preventing an incident.
I'll be on the BT myself in December, but our group will be 2 woman and 3 guys and I feel totally comfortable with that. I've been on the BT in much smaller groups before and wouldn't hesitate much to repeat that. I've been there solo too but won't repeat that in a hurry...Bell Cave is eerie at night.
So, while I don't give this a green light entirely, I would not say that it is a bad idea altogether, nor would I discourage you from going. It is a decision you must make for yourself. If you go, be careful.